Conventionally, a technique relating to a multi-layer substrate, which has excellent heat radiation property of a semiconductor element and has a low manufacturing cost, is disclosed in, for example, JP-A-2004-158545. The multi-layer substrate includes a heat radiation plate, which is disposed on both sides in an upper-lower direction and is insulated from the semiconductor element.
However, in a heat radiation structure of the semiconductor element such as a MOS-FET (i.e., metal oxide semiconductor field effect transistor), which provides a motor driving circuit, a surface mounting element such as a SMD (i.e., surface mount device) and a bare chip including a package is mounted on a surface of a substrate, and a back surface of the substrate contacts the heat radiation body so as to radiate heat. In this structure, no element is mounted on the back surface. Further, since the heat is radiated via the substrate, the heat radiation efficiency is not good.
Alternatively, a structure is proposed such that a surface mounted element having a heat radiation surface opposite to a substrate mounting surface is used, and the heat radiation surface of the surface mounted element contacts a heat radiation plate via a heat conducting member. When multiple surface mounted elements are mounted on the substrate, and a heat radiation body is attached, if the thickness (i.e., a height) of each surface mounted element is varied, the thickness of the heat conducting member is also varied. Further, since the heat radiation body is an electric conductor in general, it is necessary to secure the insulating property between the heat radiation body and the surface mounted element with the heat conducting member. Thus, it is necessary to secure the insulating property between the surface mounted element having the smallest thickness and the heat radiation body with the heat conducting member and to design thermally in view of the surface mounted element with the heat conducting member having the largest thickness. Accordingly, it is necessary to use the heat conducting member having high performance, compared with a case where a single item is attached to the heat radiation body.
The above difficulties also arise in the multi-layer substrate described in JP-A-2004-158545.